_get_host_sysinfo_serial_os fails with different exceptions if the machine-id file is not present or if it is empty

[Andrea Rosa]

When the sysinfo_serial config parameter for the libvirt driver is set to 'os', the defined behavior is:
- return the value found in /etc/machine-id file or
- return an error if the file is not present.
There is an additional scenario where the file is present but it is empty
(https://bugs.launchpad.net/ubuntu/+source/nova/+bug/1413293) in that case we want nova to behave as if the file was missed.
At the moment the missing file and an empty file return different exceptions:
- IOError for a missing file
- IndexError for an empty file
Both of these errors are too general and don't give a big help in debugging the issue.

Please note that we do not want to fix the issue about a missing/empty machine-id file, that is something related to a bad OS installation/configuration or a bad image, the proposed fix is just to keep nova behaves consistently and giving back to the user a more clear and precise error.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/202640

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/202640
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=a118c155f47711a2d4ba8aee7a6b8849a62de4bb
Submitter: Jenkins
Branch: master

commit a118c155f47711a2d4ba8aee7a6b8849a62de4bb
Author: Andrea Rosa <email address hidden>
Date: Thu Jul 16 16:12:14 2015 +0100

    Raise NovaException for missing/empty machine-id

    When the sysinfo_serial config parameter is set to 'os', Nova tries to
    get the UUID for the host operating system from the /etc/machine-id
    file. Currently if the file is missing, Nova return an IOError
    exception. If the file is present but empty, Nova will return an
    IndexError exception.

    We want to have the same behaviour in both cases and return an error
    with clearer and more useful information. This patch makes Nova return
    a NovaException when the machine-id file is either missing or empty.

    Closes-Bug: 1475353
    Change-Id: I2b163d6a42f66ab660c3728b1e29b4b64efbceba